Complex macromolecular and multimolecular systems may comprise nano-sized solute macromolecules such as proteins dispersed or dissolved in a liquid solvent, small liquid droplets or solid colloidal particles in a liquid, biological cells or membranes, and the like. An important physical property of these systems is the interaction potential between the particles or their surfaces in a suspending liquid medium or in a vapor atmosphere. The interaction potential is directly related to the interaction force between the particles or surfaces, both of which are usually formulated as functions of a separation distance, D, between the particles, or in the case of thin films, the two surfaces of film thickness, D.
The forces may be equilibrium forces (such as static or thermodynamic) or non-equilibrium forces (such as dynamic, transient, viscous, or rheological) that slowly change with time. The distances D may be measured directly, giving an absolute distance as in the surface forces apparatus (SFA) (see for example U.S. Pat. No. 5,861,954, the entire content of which is hereby incorporated by reference). Alternatively, the distance D may be measured indirectly, giving a relative or approximate distance as in the atomic force microscope (AFM).
Knowing these forces and their dynamics allows modeling of complex multicomponent systems where different molecules associate into various types of structures. These structures may then interact with each other through repulsive or attractive forces, leading to a stable dispersion of particles or an unstable dispersion where the particles slowly aggregate or coalesce and the system slowly changes or evolves over time. Biological or polymer systems composed of mixture of lipids, proteins, polyelectrolytes, and other complex molecules often have a very complex hierarchy of both their structures and their equilibration and relaxation times.
In almost every field of science, engineering, biology and medicine, there is a growing interest in understanding such systems. However, existing techniques may not be able to measure equilibrium and non-equilibrium forces in complex multimolecular and hierarchally structured systems because these techniques are often limited to measuring only one property. In addition, these techniques may be confined to measuring over a limited range of force, length, or time scales. In order to understand complex systems, it may be important to determine which molecules associate and which do not, as well as the structures that are formed by the associating molecules.
A better understanding of the structures formed by the association of complex molecules and their dynamics may be gained through the use of various types of direct or indirect visualization or imaging techniques simultaneously with measuring the equilibrium forces and/or non-equilibrium forces. These visualization and imaging techniques may include normal (optical) microscopy, fluorescence microscopy, fluorescence recovery after photobleaching (FRAP), confocal microscopy, reflective or grazing incidence x-ray scattering, and other techniques.
In addition to imaging the structures formed by the association of complex molecules, it may be beneficial to understand chemical reactions taking place between the molecules, particularly at the particle-solution interface. Such reactions may be studied by a variety of analytical techniques, such as IR-Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and energy-dispersive spectroscopy (EDS) in vacuum.
Structures may change over time as the forces within and between them change (with time). Many apparatus have been developed for directly measuring the forces between small particles or “probe tips” and surfaces (such as the AFM), and between two extended surfaces (such as the SFA). However, no single technique does both, nor allows simultaneous in situ (“real time”) imaging capabilities.